Any measurement that relates to what a user actually sees (experiences) when they use the system. User experience generally refers to end-to-end response time.
From Nielsen Norman Group∞
"User experience" encompasses all aspects of the end-user's interaction with the company, its services, and its products. The first requirement for an exemplary user experience is to meet the exact needs of the customer, without fuss or bother. Next comes simplicity and elegance that produce products that are a joy to own, a joy to use. True user experience goes far beyond giving customers what they say they want, or providing checklist features. In order to achieve high-quality user experience in a company's offerings there must be a seamless merging of the services of multiple disciplines, including engineering, marketing, graphical and industrial design, and interface design.
User experience is about usability, which has a broader scope than user interface (UI). It includes the application's look-and-feel; not just what the user-interface provides from a functional point-of-view. However, there is a component if user experience that is concerned with UI.